Antenna gain indicates how strong a signal an antenna can send or receive in a specified direction. Gain is calculated by comparing the measured power transmitted or received by the antenna in a specific direction to the power transmitted or received by a hypothetical ideal antenna in the same situation. If the comparison is to an ideal (text-book pattern, lossless) antenna radiating or receiving energy equally in all directions, the gain is measured in dBi (decibels-isotropic). If the comparison is to an ideal lossless half-wave dipole antenna, defined as having 2.15 dB gain, the gain is measured in dBd (decibels-dipole). Note that the decibel is a logarithmic unit, meaning a 6dB is almost four times the reference power; 7 dB is five times the reference power, etc.
For example, a collinear antenna oriented east-west with 6.41 dBd gain would be able to transmit or receive more than 4 times the signal power of an ideal dipole antenna in the east and west directions. There would be very little signal transmitted in the north and south directions.
Higher gain generally means that the signal is concentrated over a smaller beam width. This might be appropriate for some linear applications, such as those that need to isolate a specific signal and avoid external interfering signals. A wider beam might be needed, for example, if there are many receiving units that move around that need to stay connected, as in police or taxi dispatchers. Different antenna types have different antenna patterns.
A dipole antenna is, at its simplest, two straight rods or wires oriented end to end on the same axis, with a balanced feedline connected to the two adjacent ends. By itself, a dipole antenna is close to omnidirectional, with nearly equal power transmission in all directions. In addition to VHF transmissions, dipole antennas are often used in shortwave applications or as FM broadcast receivers.
Yagi (or Yagi-Uda)
A Yagi antenna is an array of dipole elements, where one dipole is connected to receive the signal, and the other elements are designated to reflect or redirect the energy in the desired direction. Yagi antennas are directional and need to be mounted facing the intended signal path. The main lobe is wide, with a somewhat smaller back lobe. This type of antenna can be mounted horizontally or vertically, depending on the desired signal polarization. Yagi antennas work well for point to point as well as multipoint applications.
A collinear antenna array is a series of dipoles mounted end-to-end. With up to 10dBd gain, they have a narrow band width and multiple side lobes. They are often used in mobile communications, such as police, fire, and taxi dispatchers.